In general, a quantum dot is a minute mass on the order of several to several tens of nanometers made of a semiconductor, metal, or the like, and can three-dimensionally confine electrons and holes therein. Such a confining effect quantizes the movement of electrons and holes in the quantum dot, thereby forming discrete energy levels. When a plurality of such quantum dots are arranged into an array, so as to form a quantum dot array, a quantum dot laser excellent in energy efficiency and temperature stability and the like can be realized.
As a method of manufacturing such a quantum dot array, one manufacturing a quantum dot array by patterning which employs photolithography and one manufacturing a quantum dot array by self-organization utilizing the SK (Stranski-Krasnotav) growth mode in thin-film growth have also been known.
However, the manufacturing method utilizing photolithography complicates the process and lowers the production efficiency, thereby raising the cost of devices using the quantum dot array. When cost-cutting is intended, on the other hand, the area is hard to increase.
The manufacturing method utilizing the SK growth mode utilizes the lattice constant difference between a material constituting the quantum dot and a material constituting a barrier layer, thereby limiting combinations of the materials constituting the quantum dot and barrier layer. Also, this method is hard to regulate the number density of quantum dots on a substrate. When quantum dots and barrier layers are to be laminated alternately in the film thickness direction, the barrier layers must increase their thickness in order to alleviate their lattice strain.
Therefore, a method of forming a plurality of columnar quantum dots on a substrate by obliquely vapor-depositing a material constituting the quantum dots onto the substrate has been proposed (see Suzuki and two others, “Morphological Stability of TiO2 Thin Films with Isolated Columns”, Japanese Journal of Applied Physics Part 2, Vol. 40, p. L398-L400, 2001).